Hot pressed silicon nitride containing at least a primary addition of Y.sub.2 O.sub.3, as a pressing aid has shown unusually superior results as a material useful in the cutting of cast iron. During the hot pressing, a crystalline second phase consisting of one or more of the three known types of yttrium silicon oxynitride is developed.
When cutting cast iron, a relatively large mass removal rate is experienced during machine cutting. It has been typical for the materials used earlier by the prior art, such as tungsten carbide and aluminum oxide to fail by thermal cracking. However, with hot pressed silicon nitride materials, the cutting tool can be utilized for continuous or interrupted cutting for periods in excess of 1-15 minutes and longer, whereas it is typical for a prior art tool to be utilized for 1/10 of such time. Accordingly, this new material is experiencing dramatic increases in tool life.
In spite of such increased longevity for the material from a metallurgical and wear resistant standpoint, the use of the tool has experienced one significant problem. This problem is connected with the manner in which the tool is mounted for cutting. It is typical for a metal shank to contain a pocket for receiving the cutting insert or tool. This insert is clamped in the pocket between the shank on one side and a chip breaker element on the other side. A chip breaker typically operates as a wedge in deflecting the cut chip so that they will break off from the workpiece and thereby be separated. The pocket in the tool holder is typically triangularly shaped, with the cutting insert projecting beyond the end of the shank, out of the pocket so that one point of the triangular insert is outside the sides of the shank to be free to cut; one end of the insert extends from such point in a direction generally aligned with the surface being cut. The cutting insert is held in place by frictional clamp; no holes are provided in the insert because of its ceramic nature which is notch sensitive. When the tool is withdrawn after each cycle of cutting, there may be slight frictional drag on the insert as it is pulled or dragged from the location of cutting for a repeat run. The drag tends to promote a pull on the insert causing it to pull slightly out of the pocket, allowing for debris and cut chips to become dislodged behind the insert; this eventually leads to undue forces on the insert as a result of an exaggerated overhang from the shank. The forces from the overhang position eventually result in premature fracture and failure of the tool insert with undesirable service life. This dilema of having a material which has an extremely long wear resistance and service life, but which can be mounted properly so that its service life is not prematurely cut short by unbalanced forces during machining, is a significant problem.